Understanding the redox mechanism of lithium thiophosphates as high-voltage cathode materials

High-voltage cathode materials are essential for the development of energy-dense batteries. In this work, we synthesized lithium metal thiophosphates, Li₂MP₂S₆ (M = Mn, Fe or Co), and discovered Li₂Mn_0.902P₂S₆ as a new compound. Electrochemical measurements demonstrate Li⁺ extraction from and reinsertion in Li₂FeP₂S₆ and Li₂MnP₂S₆ at ~3 V (significantly higher than other sulfide-based cathodes) with capacities of 40 and 70 mAh/g, respectively. Surprisingly, although Li⁺ was extracted from Li₂FeP₂S₆ and Li₂MnP₂S₆ at similar voltages, the redox mechanism appears significantly different between the two cations. Density functional theory calculations show that for M = Fe, it is the non-bonding Fe states that account for charge compensation of the first Li⁺ from Li₂FeP₂S₆ (traditional transition metal redox). In contrast, for M = Mn, antibonding Mn-S and S-S states are oxidized and accompanied by significant rehybridization of Mn and S states during charging. Our findings shed insight into the interplay of cationic and anionic redox in this interesting class of potential cathode materials.